1. Field of the Invention
The invention relates to film blowing apparatus wherein a polymer film is formed by extrusion through a circular die and subsequently expanded by maintaining a positive air pressure within the annulus formed by the extruded film.
2. Description of the Prior Art
Blown film processes, also known to the art as "bubble" or "tubular" film processes are well known to the art and various embodiments of such processes are in widespread use today. In such a process, a mass of molten polymeric material is forced through a circular die which die is typically equipped with or in the alternative, in close proximity to, an air blowing or air injection apparatus which is operated to provide a positive air pressure; the polymeric mass forced through and exiting the annulus of the circular die forms a continuous circular or tubular film in the form of an elongate bubble due to the positive air pressure and this circular film is typically drawn in a direction perpendicular to the circular die by some mechanical means. The film exiting the circular die is also subjected to a second stream of air on the exterior of the bubble by an air ring which is typically at or near the exit of the circular die. The air provided in the interior of the bubble second causes the expansion of the continuous film and consequently causes an increase in the circumference of the bubble, and acts to quench the film, while the second air stream quenches the film. A further effect result of this blowing procedure is the induction of orientation into the film which significantly improves the physical properties of the resultant product film.
While apparatus and methods of forming films by use of such film blowing apparatus are widespread and well known, shortcomings still exist. For example, condensation of constituents which are commonly encountered in the production of blown polymeric films, such as plasticizers, stabilizers, monomers used in producing the film, are known to occur upon surface of the air ring. Consequent to this effect, the condensed liquid upon the air ring surface is known cause spatter, and more particularly, to be entrained in the second air stream and thereby come into contact with the exterior of the bubble of blown film. The effect of such contact by the liquid of the bubble is known to introduce of undesired effects, including discoloration, degradation of the surface appearance or penetration of the film and the formation of holes in the bubble of the film.
While the production of polymeric films by blown film processes is in widespread use today, there nonetheless remains a continuing need in the art for improvements in the processes and apparatus for blown film production, and it is to this need that the present invention is addressed.